This invention relates generally to magnetic tunnel junction (MTJ) magnetic memory cells and more particularly to nonvolatile magnetic random access memory (MRAM) devices formed from an array of MTJ memory cells.
Magnetic tunnel junctions (MTJ) are promising candidates for nonvolatile memory storage cells to enable a dense, fast, nonvolatile magnetic random access memory (MRAM) array. An MTJ-based MRAM has the potential to rival conventional DRAM in density and cost, and conventional SRAM in speed. In addition, MRAM is truly nonvolatile. However, for MTJ-MRAM to replace conventional semiconductor memory technologies it is essential that the materials making up the MTJ cells be compatible with conventional complementary metal oxide semiconductor (CMOS) processing. This compatibility is necessary because the MTJ memory cells will be built on CMOS circuits which are required to read and write the state of the MTJ cells. Thus, suitable MTJ materials will be those which can successfully withstand the rigors imposed in CMOS wafer processing. MTJ-MRAM arrays are described in IBM""s U.S. Pat. Nos. 5,640,343 and 6,097,625.
An MTJ comprises two ferromagnetic layers separated by a thin insulating layer, wherein the conductance through the layers depends on the relative orientation of the magnetic moments of the ferromagnetic layers. The most useful MTJ for memory cells has the magnetic moment of one of the ferromagnetic layers free to rotate and the magnetic moment of the other ferromagnetic layer fixed or pinned by being exchange-biased with a thin antiferromagnetic layer
The main CMOS processing concern is that the MTJ materials have sufficient thermal stability to withstand back-end-of-line anneal treatments performed at a temperature in the range of approximately 350 to 400xc2x0 C. in a forming gas.
The thermal stability of the MTJ cell is affected by the cell underlayer and capping layer. The capping layer is in direct contact with the free ferromagnetic layer and thus needs to be non-interacting with the material of the free ferromagnetic layer. The underlayer is located between one of the conductor lines of the MRAM and the MTJ cell. It can be in direct contact with the conductor line or on top of a via, and thus must be capable of growing on top of copper, as well as possibly SiO2 and Si3N4or other insulating dielectrics. The thermal stability of the MTJ cell is also affected by the material chosen for the antiferromagnetic layer. While Mn50Fe50 is a common antiferromagnetic material, it does not have high thermal stability, and MTJ cells using this alloy exhibit decreased magnetoresistance (MR) when annealed at temperatures as low as 250xc2x0 C. The antiferromagnetic material is grown on the cell underlayer and thus must also be compatible with the underlayer material during CMOS processing.
Another CMOS processing concern is the requirement that the materials making up the MTJ cell are amenable to reactive ion etching and/or wet chemical etching.
A general review of materials and processes for MTJ-MRAM is presented in an on-line article by J. M. Slaughter et al, xe2x80x9cMagnetic Tunnel Junction Materials for Electronic Applicationsxe2x80x9d, JOM-e (electronic publication of The Minerals, Metals and Materials Society) 52 (6) (June, 2000).
What is needed is an MTJ cell with a set of materials that allows high MR while being fully compatible with CMOS processing steps.
The invention is a MTJ cell and a MRAM incorporating the cells. The upper and lower electrodes of each cell are formed of bilayers that provide electrical connection between the cell and the copper word and bit lines of the MRAM. The bilayers are formed of a first layer of tantalum nitride or tungsten nitride and a second layer of tantalum or tungsten. In one embodiment TaN is formed directly on the copper and low-resistivity alpha-Ta is formed directly on the TaN. If the cells use an antiferromagnetic layer to fix the moment of the pinned ferromagnetic layer, then Ptxe2x80x94Mn is the preferred material formed over the alpha-Ta. The bilayer can function as a lateral electrode to connect a horizontally spaced-apart cell and a copper stud in the MRAM. With this combination of materials each cell and thus the MRAM has high thermal stability compatible with CMOS processing.
For a further understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.